Doing it with WALTER:
One of my big goals in that whole default v6 foldering/index/indenting effort was to make it so that you could change one of these elements without it breaking the others like it would have in v5.
Code:
set tcp.trackidx + + [0 0 this_tcp_indent] [folder_sec 0 0 0 0 0 0 1] * scale [0 0 18 100]
set tcp.trackidx ?supercollapsed{0} + tcp.trackidx * scale [0 0 0 40]
set tcp_idx_margin - / h 2 * scale 18
set tcp.trackidx.margin + [this_tcp_indent] + [0 tcp_idx_margin 0 tcp_idx_margin 0.5] folderstate<0 * scale [0 -6 0 6] folderstate==1 * scale [0 16] 0
So, I only use trackidx to do x-axis work, and if you have a look at tcp_idxbg.png / tcp_idxbg_sel.png you'll see that they don't have anything going on in the y-axis. This means I can do as much walter heavy lifting in the y-axis as I like, using the margins, and its not going to affect anything else.
So : you could add an additional line for tcp.trackidx.margin, if track not selected, and use the y-margins to push the index text off the top off the panel or somewhere else its not going to be seen.
Doing it with colour:
If you know the colour of the background (so, either default colour or custom colour or some composite of the two) and you make the text that colour, the text will become invisible.
Doing this on a def6 mod with track index will depend on whether you have used my partial colouring hack, and your tolerance for it possibly looking weird if the user sets the TCP panel to an absurdly large size.